J. Helbert

South-polar features on Venus similar to those near the north pole

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright ‘dipole’ feature surrounded by a cold ‘collar’ at its north pole. The polar dipole is a ‘double-eye’ feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus’ south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

Spectroscopic Observations of Mercury's Surface Reflectance During MESSENGER's First Mercury Flyby

During MESSENGERs first flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer made simultaneous mid-ultraviolet to near-infrared (wavelengths of 200 to 1300 nanometers) reflectance observations of the surface. An ultraviolet absorption (<280 nanometers) suggests that the ferrous oxide (Fe2+) content of silicates in average surface material is low (less than 2 to 3 weight percent). This result is supported by the lack of a detectable 1-micrometer Fe2+ absorption band in high-spatial-resolution spectra of mature surface materials as well as immature crater ejecta, which suggests that the ferrous iron content may be low both on the surface and at depth. Differences in absorption features and slope among the spectra are evidence for variations in composition and regolith maturation of Mercurys surface.

A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express

The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90–120 km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft and ground-based observations of infrared emission from CO2, O2 and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission owing to a lack of data and of an adequate observing geometry. Here we report measurements of day-side CO2 non-local thermodynamic equilibrium emission at 4.3 µm, extending from 90 to 120 km altitude, and of night-side O2 emission extending from 95 to 100 km. The CO2 emission peak occurs at ∼115 km and varies with solar zenith angle over a range of ∼10 km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O2 peak emission happens at 96 km ± 1 km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted.

Optical observations of comet 67P/Churyumov-Gerasimenko

From longslit spectra obtained during the 1996 perihelion passage of Comet 67P(Churyumov-Gerasimenko, a CN production rate of (1.35 ± 0.35) x 10_25 molecules s_-1 was determined and an upper limit for the C3 production rate of 1 x 10_25 molecules s_-1 was derived. The dust color and dust activity were determined from the continuum light in the spectra. A neutral dust color was determined between 4070 A und 4600 A. The CN production rate and the Afp parameter, determined to (428.2 ± 59,8) cm were similar to the values of the 1982 perihelion passage, indicating that the comets acitivity did noch change significantly in recent orbital passages. A first crude estimate of the dust production rate indicates a high dust to gas mass ratio. R-filter images taken in March 2003 were used to study the dust coma morphology. Two jet structures were seen in the coma; these structures show no variation within the observing period from March 7 to May 30, 2003. The orientation of these stationary structures suggests an inclination of the rotation axis of the nucleus of approximately 40° with respect to the orbital plane.

The dust activity of comet C/1995 O1 (Hale-Bopp) between 3 AU and 13 AU from the Sun

The active comet C/1995 O1 (Hale-Bopp) was target of an optical long-term monitoring program carried out at the European Southern Observatory (ESO) (Rauer et al. 1997, 2003). Longslit spectra and images were obtained at heliocentric distances from 4.6 AU to 2.9 AU preperihelion and 2.8 AU to 12.8 AU postperihelion. Based on these data, the dust activity of comet Hale-Bopp is analysed. The color of the dust coma and the Afp parameter are determined. A model for the dust release from the cometary nucleus is presented and used to compute dust production rates. The dust to gas ratio is determined.

The chemistry of C2 and C3 in the coma of Comet C/1995 O1 (Hale-Bopp) at heliocentric distances rh ≥ 2.9 AU

The extraordinary activity of comet C/1995 O1 (Hale-Bopp) made it possible to observe the emission bands of the radicals C2 and C3 in the optical wavelengths range at heliocentric distances larger than 3 AU. Based on these observations, we perform an analysis of the formation of C2 and C3 in a comet coma at large heliocentric distances. We present the most complete chemical reaction network used until today, computing the formation of C2 and C3 from C2H2 ,C 2H6 ,a nd C 3H4 as their parent molecules. The required photodissociation rates of C3H2 and C3 had to be derived based on the observations. The spatial distributions of C2 and C3 calculated with the chemical model show good agreement with the observations over the whole range of heliocentric distances covered in this work. Based on the production rates for C2H2 ,C 2H6 ,a nd C 3H4, abundance ratios are obtained for heliocentric distances rh ≥ 3 AU. In comet Hale-Bopp, C2H2 and C2H6 were measured directly by infrared observations only at heliocentric distance rh ≤ 3 AU (Dello Russo et al. 2001). The model presented here greatly extends the heliocentric distance range over which hydrocarbons can be studied in the coma of comet Hale-Bopp. We discuss possible indications of these abundance ratios for the formation region of comet Hale-Bopp.

Venus Express: highlights of the nominal mission

Venus Express is the first European (ESA) mission to the planet Venus. Its main science goal is to carry out a global survey of the atmosphere, the plasma environment, and the surface of Venus from orbit. The payload consists of seven experiments. It includes a powerful suite of remote sensing imagers and spectrometers, instruments for in-situ investigation of the circumplanetary plasma and magnetic field, and a radio science experiment. The spacecraft, based on the Mars Express bus modified for the conditions at Venus, provides a versatile platform for nadir and limb observations as well as solar, stellar, and radio occultations. In April 2006 Venus Express was inserted in an elliptical polar orbit around Venus, with a pericentre height of ∼250 km and apocentre distance of ∼66000 km and an orbital period of 24 hours. The nominal mission lasted from June 4, 2006 till October 2, 2007, which corresponds to about two Venus sidereal days. Here we present an overview of the main results of the nominal mission, based on a set of papers recently published in Nature, Icarus, Planetary and Space Science, and Geophysical Research Letters.

MERTIS: reflective baffle design and manufacturing

Optical instruments for remote sensing applications frequently require measures for reducing the amount of external, unwanted stray light in the optical instrument path. The reflective planet baffle design and manufacturing process for the thermal infrared imaging spectrometer MERTIS onboard of ESAs cornerstone mission BepiColombo to Mercury is presented. The baffle has to reflect the unwanted solar flux and scattered IR radiation, and minimize the heat load on the instrument. Based on optical stray light simulations and analyses of different baffle concepts the Stavroudis principle showed the best performance and the smallest number of internal reflections. The setup makes use of the optical properties of specific conic sections of revolution. These are the oblate spheroid, generated by rotating an ellipse about its minor axis, and the hyperboloid of one sheet, obtained by the rotation of a hyperbola around its conjugate axis. Due to the demanding requirements regarding surface quality, low mass and high mechanical stability, electroforming fabrication was selected for the baffle. During manufacturing, a layer of high strength nickel alloy is electrodeposited onto a diamond turned aluminum mandrel. The mandrel is subsequently chemically dissolved. Not only the baffle, but also the baffle support structure and other mating components are electroformed. Finally, the baffle and support structure are assembled and joined by an inert gas soldering process. After the optimum baffle geometry and surface roughness has been realized, the remaining total heat flux on the baffle is only dependent on the selection of the appropriate, high reflective coating.

Deep space instrument design for thermal infrared imaging with MERTIS

MERTIS is a miniaturized thermal infrared imaging spectrometer onboard of ESAs cornerstone mission BepiColombo to Mercury. It shall provide measurements in the spectral range from 7-14 μm with a spatial resolution of maximal 300 m and 80 spectral channels in combination with radiometric measurements in the spectral range from 7-40 μm. The instrument concept therefore integrates two detector systems sharing a common optical path consisting of mirror entrance optics and reflective Offner spectrometer. Uncooled micro-bolometer and thermopile radiometer technology are implemented for lowest power consumption. Subsequent viewing of different targets including on-board calibration sources will provide the desired performance. Special attention is spent on the fully passive thermal design in the harsh environment around Mercury. The article will provide an overview of the 3 kg - instrument design and highlight the concept of the subsystems and technologies used. The status of the development process will be reported.

MERTIS-thermal infrared imaging of Mercury: advances in mid-IR remote sensing technology for planetary exploration

MERTIS (MErcury Radiometer and Thermal infrared Imaging Spectrometer) is part of ESAs BepiColombo Mercury Planetary Orbiter mission to the innermost planet of the Solar system. MERTIS is designed to identify rock-forming minerals, to map the surface composition, and to study the surface temperature variations with an uncooled microbolometer detector in the hot environment of Mercury. MERTIS is an advanced IR instrument combining a pushbroom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics and in-fight calibration components for a wavelength range of 7-14 and 7-40 μm, respectively. First results of the ongoing MESSENGER project at Mercury have shown a more complex geology and higher variability of features than previously thought. The MESSENGER studies have demonstrated the need to gain global high-resolution mid-IR spectral and temperature data to achieve a better understanding of the planetary genesis. The MERTIS measurements will acquire this currently missing data set. This article gives a summary of the instrument requirements and its design. We are reporting on the actual instrument development progress, and the status of system and subsystem qualification efforts.